Device for measuring alternating currents



s. A. HoLMQvxs'r 2,393,775 FOR MEASIJRING ALTERNTING CURRENTS DEVICEFiled oct. v14;, l1942A e E K v 0 nLTE/mr/NG- CURRENT' SOURCE Cun/zeurlsoa/ect E la , fu ri/verno SOURCE Patented Jan. 29, 1946 DEVICE FORMEASURING ALTERNATING CURRENTS Seth August Holmqvist, Gothenburg,Sweden, as-

signor to Telefonaktiebolaget L. M. Ericsson, Stockholm, Sweden, acompany of Sweden Application October 14, 1942, Serial No. 462,031 InSweden August 16, 1941 7 Claims.

The present invention relates to a device for measuring alternatingcurrent by means of one or more bridge circuits, which are passed by-apart of the alternating current as well as by a direct current servingthe purpose of bringing the bridge into equilibrium. 'Ihe invention isbased upon the use of a bridge circuit including a resistance, whichchanges with its temperature, as well as a number of constantresistances. Such a bridge circuit has the property that at a suitablechoice of the resistances the voltage drop between the ends of thebridge will -be zero only for a certain determined current intensitythrough the bridge circuit. If such a bridge circuit is traversed bydirect current as well as by alternating current and the intensity ofthe direct current is known, the effective value of the alternatingcurrent can always be determined,V if the zero current is known,independent of the frequency of the alternating current or the form ofits curve. The device according to the invention is so constructed thatit can be used for measuring current and voltage as well as the power ofalternating currents. The invention is mainly characterized by thebridge 'circuit 'being connected with resistances in such a way that itcan be connected to the alternating current circuit intended formeasuring, either in series with a resistance for measuring the voltageor in parallel with another resistance for measuring the currentintensity or in series with the former and in parallel with the latterfor measuring the power.

As an example, two devices according to the invention are schematicallyshown on the attached drawing. Fig. 1 is a switching diagram of ameasuring device according to a performance of the invention includingone bridge circuit, and Fig. 2 is a switching diagram of a measuringdevice according to the invention including double bridge circuits..

'I'he bridge circuit included in the measuring device according to thegures is composed of four resistances R1, R2, R3, R4, of which R1, R3and R4 are constant and made of manganine wire, for instance, while theresistance R2 changes with the temperature and, for instance, iscomposed of a thin wire with a high coeicient of temperature, forinstance, a platinum wire. If a current is passed through both branchesof the bridge circuit, it is evident that the galvanometer G indicateszero if R1:R3=Ra:R4. Since R1, R3 and R4 are constant and the resistancein R2 is dependent on its temperature and consequently of thel effectivevalue of the resultant current through R2, the galvanometer willindicate zero only for a certain current through the bridge circuit, thezero current. The bridge circuit is connected in series with aresistance Rs, and the bridge circuit as well as this resistance are'connected in parallel with two resistances R5 and R1 in a D. C. circuitLi-Li, which also includes a resistance R8, over which the D. C. in thecircuit Li-Li can bermeasured. The corresponding resistances included inthe second bridge circuit according to Fig. 2 have been designatedR11-R81.

In the device according to Fig. 1 the contact l in the two polechange-over switch O and one pole of a single pole switch a areconnected ata pointbetween the bridge arm R1-R4 and the resistance Re.The contacts 3 and 4 of the switch O are joined together and connectedto one pole each of two single pole switches f and e. `The other pole ofthe switch f is directly connected between the resistances R5 and R7,While the other pole of the switch e is connected with the same pointbetween theresistances R5 and R7 over a resistance R9. The switch O isof the two pole change-over type which when thrown to the left connectsthe contacts 2 and 5 with the contacts I and 4 and when thrown to theright connects the contacts 2 and 5 with the contacts 3 and 6. Aterminal 1 is connected to the second mentioned pole of the switch fover a short-circuiting resistancev R10, which is suitably adjustable toits capacity for dilerent measuring ranges. Another terminal 8 isconnected between two resistances R11 and R12, of which the former isconnected to a contact 2 in the switch O as well as to a terminal 9,while the resistance R12 is connected to the one contact in a switch b,the second contact of which is connected to a contact 5 in the switchsituated opposite the contact 2. The contact 6 in this switch isconnected with the switch a. Between the terminal 9 and a terminal l0 ashunt resistance R13 is connected and the terminal Ill can be directconnected to contact 5 over a switch c as well as over a resistance R14to the same contact over a switch d. The resistances R13 and R14 shouldsuitably be adjustable for dif-l y ferent measuring ranges.

The device according to Fig. 2 diiers from that in Fig. 1 by theresistance R9 being replaced by another direct current circuit Lz-Lzincluding the resistances R11-Ra1 in the same connection as theresistances Ri-Rs in the circuit L1-L1 and further by the elimination ofthe change over switch O` and the switch a.

When the devices described .above are to'ybe used, the zero current ofthe bridge circuit is at 27Min,

' Y first determined; i. e., the current at which the galvanometer Gdoes not give any deviation. At this current intensity the voltage V ismeasured Yover the resistance Rs, respectively R81., from when me deviceaccording to Fig. 1 is to be used for measuring a voltage, the desiredalternating current voltage V is connected between the terminalsv 1 and8 and the switch is thrownY to the left connecting contacts `2 and 5with con'- tacts I and 4, whereupon the direct current in the circuitL1-L1 is so adjusted that the vgalvanom eter G again indicates zero.Over the resistance Ra the direct current required for bringing thebridge into equilibrium can b'e read by rneasur The desired effectivevalue Vc of the alternating current voltage is thenl oblng the voltageV1.

tained according to the formula Y V.=K.Vv0"2 V12 where K is a constantdepending on R10.

When measuring alternating current intensi'- ties with the deviceaccording to Fig. 1 the alternating current is connected tov theterminals 9 and vl 0, whereupon the contacts c and f are closed andtheswitch O thrown into the left position. In parallel with the resistanceR13 a circuit is then closed over the switch c, contact 5, contact 4,switch f, bridge circuit, contact I, contact 2 to terminal 9. The directcurrent in the circuit L1-L1 is then adjusted, so that the galvanometerG indicates zero, determining the voltage V1 over Rs. The desiredeffective value -Ic of the alternating current is obtained according tothe formula where C is a constant depending on R13.-

The determination of active alternating current power can be carried outwith the device shown partlyby means of different current and voltagecircuit (supervision of power and energy meters)` partly by a commonpower source (the measuring of the power consumption of a device).

When measuring the power with the help `of separate current and voltagecircuits, alternating rvoltage and current are connected to the termi'-nals l, 8, and 9, I0.' The switches a, b, c, and e are closed and theswitch O is thrown to the left. When the direct current through L1--L1has been adjusted, so that the galvanometer G indicatesP'==K.C(V12-V2f2) watt active power where K and C are constants, whichare only depending on R10 and R13.

MFor a direct measure of the power, the power consuming instrument isconnected between the terminals 'I and 9 and the power supply to theterminals 1 and I0. Consequently, the resistance R13 is connected inseries with the power consuming instrument and the bridge is connectedin parallel to said instrument. The switches a,

d and e are closed, and the switch O is thrown to the left, theD. C.through L1-L1 is adjusted so that the galvanometer G indicates zero, thevoltage V1 is measured. The switch O is then thrown to the right, vtheD. C. through L11-L1 again adjusted so that the galvanometer G indicatesZero, and the voltage V2 is measured, whereupon the power consumed bythe instrument is calculated in the same way as described above.

When using the measuring device according to Fig. 2 for measuringcurrent or voltage, V1 is, for instance, measured over Rc "and Vc overR81.

'When measuring the power7 V1 is `measured over the resistance Rs at theone bridge circuit and V2 over the resistance R81 at the other. The D.C; circuits L1L1 and Lz-'Lz are adjusted for the same zero current. Thedesired yvalues of the alternating current are obtained in the same wayas describedabove, but the handlingof the device f The performances ofthe invention described above and shown on the drawing are only to beconsidered as examples, and the details of the invention can be modiedin diierent'manners within the limits of the patent claims. The switch Oshown inFig. 1 can thuspossibly be replaced by a switch inserted inthevoltage circuit, serv'- ing the purpose Yof changing the phase angle-for the vvoltage instead of the current when measuring the power.v Withthe device accord'- ing to Fig. V2 the connection for measuring thepower can be made in such a way that the alternating voltage Vwill havethe same phasekposition in both bridge circuits but that instead thephase angle for the alternating current-Will be'changed 180 in thedifferent bridgecircuits.

I claim:

1. In an alternating current meter, iirst andV second bridges, the rstbridge forming part of the second bridge and including a resistanceelement varying in Value in dependence Vupon the temperature thereof, adirect current source confnected with two opposed points of said secondbridge, means for varying the direct current intensity for balancingthetwo bridges, means for indicating the balance of the two bridges,`i'lrst 'and second. resistances connected together in series andconnected in parallel to two other opposed points of the second bridge,a third resistance connected at one end with the one of the two secondmentioned opposed bridge points with which the second resistance isconnected, and means for changing the connections of the rst resistance.

2. In an alternating current meter, rst and second bridges, the iirstbridge forming part of the second bridge and including a resistanceelement varying in value in dependence upon the temperature thereof, adirect current 'source connected with two opposed points vof said secondbridge, means `forvarying the direct current inten'sity for balancingthe two bridges, means for indicating the balance of the two bridges,flrst and second resistances connected `together in series and connectedin parallel to two Yother opposed points of the second bridge, a thirdresistance connected at one end with the one of the two second mentionedopposed bridge points with which the second resistance is corlliected,A

first and second terminals, the iirst terminal being connected with theopposite end lof the thirdresistance and the second termina1 beingconnected with the mid point of the rst resist.- ance, and means forchanging the connections of said two terminals.

3. In an alternating current meter, iirstand second bridges, the rstbridge forming part of the second bridge and including a resistanceelement varying in value in dependence upon the temperature thereof, adirect current source connected with two opposed points of said secondbridge, means for varying the direct current intensity for balancing thetwo bridges, means for indicating the balance of the two bridges, rst,second and third resistances connected together in series and connectedin parallel to two other opposed points of the second bridge, a fourthresistance connected at one end with the one of the two second mentionedopposed bridge points with which the third resistance is connected, andmeans for changing the connections from the rst and second resistancesto said third resistance and said second bridge, respectively.

4. An alternating current meter as claimed in claim 1 wherein means isprovided for connecting the two second mentioned opposed bridge pointsto an alternating current circuit either in series with the thirdresistance for measuring the voltage of said alternating current circuitor in parallel with said rst resistance for measuring the alternatingcurrent intensity, or in series with the third resistance and at thesame time in parallel with a series containing two equal resistances andsaid second resistance for measuring the power passing said alternatingcurrent circuit.

5. An alternating current meter as claimed in claim 2 wherein means isprovided for connecting the two second mentioned opposed bridge pointsto an alternating current circuit either in series with the thirdresistance for measuring the voltage of said alternating current circuitor in parallel with said nrst resistance for measuring the alternatingcurrent intensity, or in series with the third resistance and at thesaine time in parallel with a .series containing two equal resistancesand said second resistance for measuring the power passing saidalternating current circuit.

6. In an alternating current meter, two first and two second bridges,each rst bridge forming part of one of the second bridges, each rstbridge including a resistance element varying in Value in dependenceupon the temperature thereof, two direct current sources each connectedwith two opposed points of one of the second bridges, means for varyingthe direct current for balancing the four bridges, means for indicatingthe balance of the bridges, means connecting two other opposed points ofthe two second bridges in series,.afrst resistance connected in parallelto the second mentioned points, and a second resistance connected to theconnecting point between the second mentioned bridge points.

7. In an alternating current meter, two first and two second bridges,each rst bridge forming part of one of the second bridges, each firstbridge including a resistance element varying in value in dependenceupon the temperature thereof, two direct current sources each connectedwith two opposed points of one of the second bridges, means for varyingthe direct current for balancing the four bridges,'means for indicatingthe balance of the bridges, means connecting two other opposed points ofthe two second bridges in series, first and second resistances connectedtogether in series and connected in parallel to the second bridgepoints, and a third resistance connected to the connecting point betweenthe second mentioned bridge points.

. SETH AUGUST HOLMQVIST.

